1. Field of the Invention
The present invention relates to an image forming apparatus which includes a fixing portion having an endless belt, a heater disposed in contact with the inner surface of the endless belt, and a pressure roller defining a fixing nip portion with the heater via the endless belt, and forms a toner image on a recording material.
2. Description of the Related Art
A thermal roller fixing system or a film heating system has been conventionally adopted as heating/fixing portion for a copying machine or a printer of an electrophotographic system. In particular, the film heating system enables a fixing member to speedily rise up in temperature during printing while suppressing power consumption on standby to a low level, and therefore, has attracted attention as a heating system of an energy saving type.
A fixing device (an on-demand fixing device) of the film heating system is disclosed in, for example, Japanese Patent Application Laid-open (JP-A) Nos. 63-313182, 2-157878, 4-44075 to 4-44083, and 4-204980 to 4-204984. FIG. 13 illustrates a typical example of the fixing device of the film heating system. A film assembly 60 is illustrated in FIG. 13. A heater 61 having an energization heat generation resistant layer formed on a ceramic substrate made of alumina or aluminum nitride is fixed to a stay holder 62 made of a heat resistant resin. The fixing device includes a heat resistant, thin fixing film (i.e., an endless belt) 63 which is loosely fitted around the stay holder 62 and is made of a resin such as polyimide or metal such as SUS (stainless).
The heater 61 in the film assembly 60 and a pressure roller 50 are brought into press-contact with each other while holding the fixing film 63 therebetween, thereby defining a fixing nip portion.
The pressure roller 50 includes an elastic layer 52 made of silicone rubber and a toner parting layer 53 made of a fluorocarbon resin around a cored bar 51. The fixing film 63 is conveyed and moved in a direction indicated by an arrow while sliding in close contact with the heater 61 at the fixing nip portion by the rotational driving force of the pressure roller 50 in the arrowed direction. The temperature of the heater 61 is detected by a thermistor 64 disposed on the back of the heater, to be fed back to an energization control portion, not illustrated, so that the heater 61 is heated and adjusted so as to be kept at a predetermined temperature (i.e., a fixing temperature). An image forming apparatus using the fixing device of the film heating system has a high heating efficiency and speedily rises up in temperature, and thus, has more advantages, such as the lack of any need preliminary heating on standby and a shortened waiting time, than an image forming apparatus using a conventional fixing device of a thermal roller system.
The pressure provided by the fixing device needs to be normally increased during printing. However, the pressure needs be set to a lower level in order to readily remove a recording material nipped at the fixing nip portion when the recording material staying at the fixing nip portion due to paper jamming is removed (hereinafter it is referred to as jam recovery). Otherwise, when the image forming apparatus is left unused for a long period of time or before the image forming apparatus is shipped, the pressure needs be set to a lower level in order to prevent deformation of the pressure roller and the endless belt. As a countermeasure against such a problem to be solved, the jam recovery is improved or the deformation of the endless belt and the pressure roller is prevented by setting the pressure to be exerted on the fixing nip portion to a lower level during the jam recovery or non-use in comparison with during normal printing, as disclosed in JP-A No. 2000-122460.
However, a heater may not be bonded to a heater holder in order to prevent a crack of the heater due to a difference in thermal expansion between the heater and the heater holder in the heating/fixing device of the film heating system. With such a configuration, a lower limit of the pressure is restricted during the jam recovery. That is to say, the heater is separated from the heater holder under a too low pressure, so that an edge of the heater projects from the heater holder. If the jam recovery is performed in such a state, the inner surface of the fixing film is damaged by the edge of the heater when the fixing film is rotated. When the heating/fixing device is continuously used while the inner surface of the fixing film is damaged, the fixing film may be possibly broken. To the contrary, when the fixing film and the pressure roller are completely separated from each other such that the fixing film cannot be damaged even during the jam recovery, a space for a pressure mechanism must be sufficiently provided.
Alternatively, when the image forming apparatus is stored for a long period of time while the pressure is set such that the edge of the heater cannot project from the heater holder, a phenomenon of deformation of the elastic layer of the pressure roller or the fixing film (hereinafter referred to as “an elastic layer deformation phenomenon”) may occur. The elastic layer deformation phenomenon induces deformation of specific portions of the fixing film and the pressure roller that are press contacted for a long period of time, thereby raising a problem of degradation of uniformity of an image at the deformed portion.
As described above, when the pressures during the jam recovery and the non-use are set to the same level, it may be difficult to prevent the breakage of the fixing film and the deformation of the pressure roller or the fixing film at the same time.